While lead-free solder satisfies the requirement of environment protection, a better thermal stability of the soldering iron is also required. The common soldering iron always uses a linear transformer as the power supply circuit. In this case, the output voltage is constant, and the output power is deficient at high temperature. As a single absolute temperature control manner is used as the temperature control manner, in the process of jointing, the rate of re-rising the temperature is slow, and a cold joint or rosin joint is inclined to take place. Some electric soldering irons use absolute temperature control manner to control the temperature-power of the electronic heating element, that is, only sample and control the absolute temperature of the heating element, but cannot determine the operation status of the heating element and the thermal requirement of the heated object and its effect to the temperature. When the temperature is falling, the transient response of the heating element is poor, thus the re-rising of the temperature is slow, and this cannot be used in the case that the heat transfer of the heating element changes rapidly. For example, temperature of the electric soldering iron is very high when it does not touch the metal to be welded, once it touches a big-size metal object or keeps contacting with the metal object for a long period of time, temperature of the electric soldering iron may fall rapidly due to the fast heat transfer. Moreover, the cold joint and rosin joint is especially easy to be induced in the case that the metal to be welded has a large surface or the environmental temperature is low. In order to detect the change status for the load to the soldering iron head, the nice difference of the temperature should be obtained in the shortest time. The earlier the detection occurs and the earlier the control performs, the quicker the induced response of the temperature is. However, the earlier the detection occurs, the weaker the detected signal is. And the detection should be ensured away from the affect of the interfering noise outside. Since the heating element and the temperature sensor element are all installed in the soldering iron head, the electronic signal from the temperature sensor element is delayed with respect to the actual time of the temperature change happened to the soldering iron head, and the time of the power change happened to the heating element is also delayed with respect to the actual time of the temperature change happened to the soldering iron head. It is crucial that how the factor of delay, the interference, and the requirement of response time should be considered in designing an electric soldering iron.